In recent years, as a method for manufacturing a bonded wafer, a method for delaminating an ion-implanted bond wafer after bonding to manufacture a bonded wafer (an ion implantation delamination method: technology that is also called a smart cut method (a registered trademark)) has newly started to attract attention. This ion implantation delamination method is technology that forms an oxide film on at least one of two wafers, implants gas ions such as hydrogen ions or rare gas ions from an upper surface of one wafer (a bond wafer), forms a micro bubble layer (a sealing layer) in the wafer, then adhering the ion-implanted surface to the other wafer (a base wafer) directly or through an oxide film (an insulator film), subsequently performs a heat treatment (a delamination heat treatment), uses the micro bubble layer as a cleaved surface to delaminate one wafer (the bond wafer) in a thin film shape, and further performs a heat treatment (a bonding heat treatment) to strengthen the bonding, thereby manufacturing a bonded wafer having a thin film on the base wafer (see Patent Literature 1). According to this method, the cleaved surface (a delaminated surface) is an excellent mirror surface, and an SOI wafer having a thin film or an SOI layer in particular whose film thickness uniformity is high to some extent is easily obtained.
However, in case of manufacturing the bonded wafer based on the ion implantation delamination method, a damage layer caused due to ion implantation is present on the bonded wafer surface after delamination, and surface roughness is higher than that of a mirror surface of a silicon wafer that is at a regular product level. Therefore, in the ion implantation delamination method, such a damage layer or surface roughness must be removed.
In conventional examples, to remove this damage layer or the like, mirror polishing (a stock removal: approximately 100 nm) with a very small polishing stock removal, which is called touch polish, is carried out at a final step after the bonding heat treatment.
However, when the thin film on the base wafer is subjected to polishing including machine work, since the polishing stock removal is not uniform, there occurs a problem that film thickness uniformity of the thin film achieved to some extent by implantation of hydrogen ions or the like and delamination is deteriorated.
As a method for solving such a problem, a flattening treatment for performing a high-temperature heat treatment in place of the touch polish to improve surface roughness has been carried out.
For example, Patent Literature 2 suggests adding a heat treatment (a rapid heating/rapid cooling heat treatment (RTA)) in a reducing atmosphere containing hydrogen without polishing a surface of an SOI layer after a delamination heat treatment (or after a bonding heat treatment). Further, Patent Literature 3 suggests forming an oxide film on an SOI layer by a heat treatment in an oxidizing atmosphere after a delamination heat treatment (or after a bonding heat treatment), then removing the oxide film, and subsequently adding a heat treatment (the rapid heating/rapid cooling heat treatment (RTA)) in a reducing atmosphere.
Furthermore, in Patent Literature 4, after a flattening heat treatment in an inert gas, a hydrogen gas, or a mixed gas atmosphere containing these gases, a sacrificial oxidation treatment is performed with respect to an SOI wafer after delamination to achieve both flattening of a delamination surface and avoidance of OSF.
Since the flattening treatment for improving surface roughness by carrying out the high-temperature heat treatment in place of touch polish is performed in this manner, each SOI wafer that has a diameter of 300 mm and also has film thickness uniformity that a film thickness range of an SOI layer (a value obtained by subtracting a minimum film thickness value from a maximum film thickness value in a radial direction) is 3 nm or less is mass-produced by the ion implantation delamination method.